Extrusion of low density foams is well known in the art. These foams are prepared by advancing a particulate resin through an extruder where it is melted and then injected with a blowing agent. As the mixture of the molten resin and the blowing agent is further conveyed through the extruder, it is mixed, cooled to reduce vapor pressure of the blowing agent and to increase viscosity of the molten resin in order to promote formation of closed cells, and extruded through a die into a zone of lower pressure whereby the blowing agent expands to form a low density foam product.
In the prior art practice, blowing agent is introduced into molten resin through a single port with the result that there is inadequate distribution of the blowing agent within the resin. To overcome this problem, a number of approaches have been tried to promote uniform distribution of the blowing agent within the resin, including the provision of a longer agitation zone, innovative designs of feed screws in the extruder, etc. It is believed that the invention disclosed herein facilitates distribution of the blowing agent in the resin in a novel way that does not require elongated agitation zone or a unique design of the feed screw although these features can be used in conjunction with the invention disclosed herein to further improve uniform distribution of the blowing agent within the resin.